1. Field of the Invention
The present invention relates to a system for particle beam therapy and, in particular, concerns a system for producing a beam comprising multiple types of particles, including protons and neutrons, which are delivered to a target isocenter within the patient to be treated.
2. Description of the Related Art
Radiation therapy is used to treat, among other things, various forms of cancer. In particular, radiation therapy is often used to treat tumors and cancerous tissues within the body of a patient. Generally, radiation therapy involves directing a beam of sub-atomic particles at a region within the body of the patient containing the cancerous tissue. Ideally, when the sub-atomic particles contact the cancerous tissue, the energy of the sub-atomic particles destroys these cells or prevents these cells from reproducing or growing. Various types of sub-atomic particles are used in various forms of radiation therapy including, photons (X-rays), neutrons and protons. Each form of particle therapy has certain advantages and disadvantages.
For example, neutrons have a comparatively high Radio Biologic Effect (RBE), which is an empirically measured value indicative of the amount of damage to cells that is caused by the particle. Hence, neutrons can be very effective in destroying or damaging cancerous cells. However, it is generally very difficult to control a neutron beam so as to direct neutrons from a remote source into particular localized regions of a patient's body. Further, as neutrons travel through the body, they release energy at a generally constant rate along their path of travel. Neutrons deposit energy along their path and are also slowed significantly as a result of penetrating tissue. Neutrons have the unfortunate tendency to damage healthy tissue surrounding the tissue of interest, i.e., healthy tissue along the path of travel of the neutrons. Additionally, neutrons present a particular safety hazard in that they can penetrate deeply into almost any type of material including steel and concrete. Thus, if large quantities of neutrons are generated, radiation shielding becomes a significant problem.
In contrast, protons generally have a lower RBE. However, they are easier to control and exhibit a characteristic known as the Bragg peak whereby the energy released by a proton penetrating tissue rapidly increases when the proton slows to a threshold level. Consequently, by controlling the energy of the protons so that they slow down at the proper time, it is possible to cause most of their energy to be released in the proximity of the region of the cancerous cells. Protons are, therefore, very effective in destroying cancerous cells within the body of a patient while minimizing damage to surrounding cells.
While proton therapy can be very effective in some circumstances, there are sometimes cases where the RBE of a beam of protons may be insufficient to destroy the cancerous cells within a particular region. In these cases, proton radiation therapy is typically discontinued in favor of other therapies.